Once you've gotten started, it's not really necessary to set aside anything. Like MMM, we have a huge buffer and it would not matter much if my car exploded. I mean, it would be a setback, but we have plenty of money in the emergency fund to cover it. In fact, I'd view it as an opportunity to make an even more frugal choice on the next one.
If your emergency fund is not large enough to cover such an eventuality, then you should be growing it.